The idea that there may be genetic differences in intelligence between one population and another has resurfaced recently, notably in the form of a New York Times op-ed by the Harvard geneticist David Reich. In the article, Reich emphasises the arbitrary nature of traditional racial groupings, but still argues that long periods of ancestry on separate continents have left their genetic marks on modern populations. These are most evident for physical traits like skin and hair colour, where genetic causation is entirely uncontroversial. However, Reich asserts that all genetic traits, including those that affect behaviour and cognition, are expected to differ between populations or races.

This extrapolation from the genetics of physical traits to how our brains work brings back memories of an argument made by the US researchers Charles Murray and Richard J Herrnstein in their 1994 book The Bell Curve, recently resurrected by Murray in conversations with the US neuroscientist and author Sam Harris. In the book, Murray and Herrnstein claim that observed differences in the mean IQ scores of ethnic groups are “highly likely” to be due to both environmental and genetic factors. This sounds quite reasonable at first: the argument concedes that environmental and cultural factors play a big part in any differences seen in the mean IQ scores of various groups. But it also suggests that since genetic variation will contribute to higher or lower IQ in any given population, the genetic differences between one group and another will also underpin mean differences in IQ.

In fact, the genetics and evolutionary history of intelligence suggest just the opposite. Most of our traits, such as height, for example, are set by natural selection at an optimal level – it’s good for humans to be about so tall, on average. Some genetic variants tend to make people a bit shorter than average and some tend to make people a bit taller. The balance between these variants has been maintained by natural selection to keep average height “just right”. Intelligence is not like that. Unlike height, where being ever taller had no benefit, strong evolutionary forces drove intelligence in one direction only in our ancient ancestors.

Intelligence is our defining characteristic and our only real advantage over other animals. It gave us an initial leg-up in colonising diverse environments and its usefulness was massively amplified by the invention of culture and language. This increasing selective advantage of ever greater intelligence led to a snowball effect, which was probably only stopped by the limitations of the size of the birth canal and the metabolic demands of a large brain.

Evolution thus endowed us with a genetic program that holds the instructions of how to build our complex brains, with our resultant cognitive prowess. But any genetic program will be affected by chance mutations and this one is no different. What sets it apart from traits like height is that most genetic random mutations that affect on intelligence will do so negatively.

Statistically speaking, random mutations are vastly more likely to mess up the complicated genetic program for brain development than improve it, especially in ways that natural selection has not already fixed in our species. For the same reason, random tinkering with the highly tuned engine of a Formula One car is vanishingly unlikely to improve performance. Similarly, we shouldn’t expect intelligence to be affected by a balance of IQ-boosting mutations and IQ-harming mutations. Instead, genetic differences in intelligence may largely reflect the burden of mutations that drag it down.

Because most random mutations that affect intelligence will reduce it, evolution will tend to select against them. Inevitably, new mutations will always arise in the population, but ones with a large effect on intelligence – that cause frank intellectual disability, for example – will be swiftly removed by natural selection. Mutations with moderate effects may persist for a few generations, and ones with small effects may last even longer. But because many thousands of genes are involved in brain development, natural selection can’t keep them all free of mutations all the time. It’s like trying to play multiple games of Whack-a-mole at once, with only one hammer.

The result is that any population at any time will carry a varied bunch of mutations that affect intelligence. These will differ between populations, clans, families, and individuals. This constant churn of genetic variation works against any long-term rise or fall in intelligence.

Another crucial point is that genetics tends to affect intelligence in a much more indirect way than it does skin colour, height, and other physical traits. Like that Formula One car’s performance, intelligence is an emergent property of the whole system. There is no dedicated genetic module “for intelligence” that can be acted on independently by natural selection – not without affecting many other traits at the same time, often negatively.

We need to get away from thinking about intelligence as if it were a trait like milk yield in a herd of cattle, controlled by a small, persistent and dedicated bunch of genetic variants that can be selectively bred into animals from one generation to the next. It is quite the opposite – thousands of variants affect intelligence, they are constantly changing, and they affect other traits. It is not impossible for natural selection to produce populations with differences in intelligence, but these factors make it highly unlikely.

To end up with systematic genetic differences in intelligence between large, ancient populations, the selective forces driving those differences would need to have been enormous. What’s more, those forces would have to have acted across entire continents, with wildly different environments, and have been persistent over tens of thousands of years of tremendous cultural change. Such a scenario is not just speculative – I would argue it is inherently and deeply implausible.

The bottom line is this. While genetic variation may help to explain why one person is more intelligent than another, there are unlikely to be stable and systematic genetic differences that make one population more intelligent than the next.

So if we are concerned about people’s intelligence, we would do better to focus on the environmental and cultural factors that we know are involved and which can be changed. There is no shortage of them: maternal and infant healthcare, early life nutrition, exposure to neurodevelopmental toxins such as lead, and access to and quality of education all make a real difference. IQ scores are a measure of a person’s intellectual ability, not the limit of their intellectual potential. Focusing on things we can change should ensure that everyone can reach their potential.